1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer, and more particularly to an apparatus which cleans a transfer member that transfers a toner image from an image bearing member to a recording material.
2. Description of the Related Art
In an image forming apparatus, there is an increasing demand for the density/color quality to look the same from a first image to a last image when a plurality of the same images is output. A configuration that an adjustment toner image (a patch) is formed at a non-image position and image control is carried out has been conventionally known. For example, there is also known an apparatus that forms a patch between an image and an image (a sheet gap) and is disclosed in Japanese Patent Application Laid-Open No. 2003-202711. Further, as disclosed in Japanese Patent Application Laid-Open No. 2006-91179 and Japanese Patent Application Laid-Open No. 2007-47554, there is also known an apparatus that forms a patch outside an image area in a direction orthogonal to a moving direction of an image bearing member.
This patch is formed on an image bearing member such as a photosensitive drum or an intermediate transfer belt, and then passes through a position at which a transfer member such as a transfer roller, which is configured to transfer a toner image from the image bearing member onto a recording medium, is present. In this case, there has been conventionally known an apparatus that keeps the transfer member away from the image bearing member so that the patch cannot be transferred onto the transfer member. Furthermore, as disclosed in Japanese Patent Application Laid-Open No. 2004-309696 and Japanese Patent Application Laid-Open No. 2008-89657, there is also known an apparatus that clean off toner adhering to a transfer member without separating the toner member.
For example, a fur brush abutting on a transfer roller is provided, and a bias having a polarity opposite to that of toner is applied to the fur brush through a bias roller, whereby the toner adhering to the transfer roller is transferred to the fur brush. Moreover, the toner that has transferred to the fur brush is further transferred to the bias roller, and the toner adhering to the bias roller is scraped off using a blade or the like.
When forming a patch image outside an image area in a direction orthogonal to a moving direction of an image bearing member, a toner image in the image area is transferred onto a recording material and, at the same time, patch toner is transferred onto a transfer member. That is, the patch image is formed outside the image area in the direction orthogonal to the moving direction of the image bearing member. Therefore, when a transfer voltage is applied to the transfer member in order to transfer an image onto the recording material, the patch toner present in a region where the recording material is not interposed is directly transferred onto the transfer member at the same time. The patch toner transferred onto the transfer member is collected by the fur brush when a bias having a polarity opposite to that of the toner is applied to the fur brush. However, the toner may not be sufficiently collected depending on a bias that is applied to the fur brush.
Usually, an absolute value of a transfer bias for thick paper is set larger than that for plain paper. Therefore, the thick paper has a higher current flowing through a patch image portion present in a region where the paper is not interposed on the outer side along the longitudinal direction of the recording material than the plain paper in dependence upon a set voltage. In case that the toner is electrically charged to have the negative polarity, when a transfer current having the positive polarity is large, a change amount of the toner having the negative polarity is shifted to the positive polarity which is the opposite polarity, and the charge amount of the toner is reduced. Therefore, when a current value applied to the fur brush is set constant irrespective of a transfer voltage, an appropriate electric field associated with the charge amount of the patch toner is not formed. For example, when the current value is too small for the charge amount of the patch toner, the electric field is insufficient, and the toner cannot be sufficiently collected to the fur brush. When the current value is too large for the charge amount of the patch toner, the polarity of the toner is reversed, and the toner cannot be likewise sufficiently collected to the fur brush. If the toner is not sufficiently collected from the transfer member, a back side of the recording material that passes through the transfer member is contaminated.